Release of Alcohols Through Photoenolization
In this work photochemistry of several types of o-alkylphenyl ketones containing an ester moiety is described. These ketones undergo photoenolization upon irradiation with light and the photoenols can release the alcohol moiety via intramolecular cyclization hereby acting as photoremovable protecting groups for alcohols. Photolysis of Benzoates I in various solvents and thin films results in liberation of the alcohol moiety from the ester. The photorelease mechanism has been established by laser flash photolysis and product studies. Upon irradiation with light Benzoates I undergo photoenolization giving a short-lived Z photoenol and long-lived E photoenol. The E photoenol than undergoes intramolecular cyclization releasing the alcohol moiety whereas the Z photoenol reverts back to the starting material. The release occurs independently from the reaction media and in thin films. The triplet excited state of Benzoates I is too short-lived and is not quenched by molecular oxygen. When irradiated in the presence of oxygen, the photorelease occurs but the main product is an endoperoxide resulting from trapping of the triplet biradical and the photoenols with molecular oxygen. Benzoates II do not release the alcohol moiety upon irradiation with light. The E photoenol was measured to be too short-lived by laser flash photolysis to undergo intramolecular cyclization and reketonizes efficiently by acid catalysis through the solvent. This reketonization is facilitated by intramolecular H-atom bonding. Photorelease also does not occur upon irradiation with light in the presence of oxygen, since the initially formed endoperoxide rearranges with release of water to give either an aldehyde or ketone. When irradiated in proton donating solvents such as 2-propanol they gave product coming from photoreduction of the ketone carbonyl by the solvent. Oxobutanoates I released the alcohol when irradiated with light and gave the expected benzcyclobutane photoproduct. Oxobutanoates I have an alkyl chain as the ester moiety instead of the benzoate phenyl ring as in Benzoates I which was designed to make the photoenols less rigid. The photorelease mechanism is similar to that for Benzoates I. When irradiated with light in the presence of oxygen, they reacted the same way as Benzoates II. No release of alcohol occurred and the products coming from rearrangement of an intermediate endoperoxide were isolated. Oxobutanoates II did not release alcohol upon irradiation. No photoproducts were isolated that would come from intramolecular reactivity of the photoenols. The photoenols or the triplet biradical could not be trapped with molecular oxygen or other dienophiles. In the presence of oxygen the main photoproduct was a lactone whose formation by a photochemical pathway has not been described in the literature.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:photoremovable protecting groups for alcohols photoenolization of o alkylphenyl ketones
Date of Publication:01/01/2005